1. Field of the Invention
This invention relates to a bypass pair control apparatus for a thyristor bridge used in an AC/DC power coupling system of, for example, a DC power transmission system.
2. Description of the Related Art
In an AC/DC coupling system using power converters constructed by thyristor bridges, the control system thereof may sometimes effect an operation called a bypass-pair (hereinafter referred to as BPP) when the system is triggered or stopped or an accident has occurred therein (cf. Japanese Patent Publication (koukoku) No. 62-49828, for example). A BPP indicates a condition that a high voltage side valve (e.g., X-phase valve) and a low voltage side valve (e.g., U-phase valve) of the thyristor valves, which constitute the thyristor bridge of the power converter and which are connected to the same phase (e.g., R phase), are set in the conduction state at the same time.
There are some methods of determining a valve which is set to be a BPP. For instance, a BPP can be set by applying a firing pulse to a valve which comes next but one to a valve which has been supplied with a firing signal (PHS signal) in the last cycle according to a normal firing sequence before the BPP is set up. Such BPP setting is explained with reference to FIGS. 1A and 1B. More specifically, if the valve supplied with the PHS signal in the last cycle is valve 2W of FIG. 1A, a PHS signal is supplied to valve 2U to trigger valve 2U of FIG. 1B. Then, valves 2U and 2X are set into the BPP.
Further, in order to prevent the entire system from being made inoperative when an accident has occurred in the control system, two series of control systems are often used (duplex system) to enhance the reliability of the system operation.
However, when a duplex system containing A and B series is adapted to the control system, the following problem may occur. Control systems A and B are basically of the same construction. However, because of variations in the characteristics of electrical parts used in a phase control circuit and PLL circuit constituting each of the control systems, a difference will occur in the generation timing of PHS signals which are the outputs of the control systems.
Therefore, it is possible that control system A outputs a PHS signal to valve 2W immediately before the BPP operation is started, but control system B does not output a PHS signal to valve 2W. At this time, control system A tends to set valves 2U and 2X into the BPP operation, but control system B tends to set valves 2V and 2Y into the BPP operation.
In the above case, the actual BPP valves do not match the corresponding bypass-pair command generated by either of the control systems. Otherwise, there may occur a problem that the BPP operation is not started and valves 2U and 2Y or valves 2V and 2X are kept in the conduction state.